1. Field of the Invention
The present invention relates to a heat generator, into which heat generator a medium flows through a flow duct during operation, the flow duct having at least one discontinuous cross-sectional expansion in the direction of a main flow in such a way that at least one wall bounding the flow duct has a step extending substantially transversely to the main flow direction.
2. Discussion of Background
In combustion technology, it is frequently necessary to operate with widely varying flow velocities. Whereas, for reasons of flame stability, the flow velocity in the heat generators themselves is limited to quite low values, various reasons often make it necessary to provide the inlet flow to the heat generators with high velocities. Because of the demands made on the installation size, it is usually impossible to decelerate the inlet flow to a heat generator in a continuous manner. In consequence, sudden-expansion diffusers with discontinuous cross-sectional expansions are very frequently employed. Although these cause substantial losses in total pressure, they provide a very compact installation. In addition, reverse flows generated in sudden-expansion diffusers are quite desirable, particularly for flame stabilization in heat generators.
However, the vortex structures which occur in sudden-expansion diffusers can also involve extremely damaging consequences under certain circumstances, particularly where the sudden-expansion diffuser is designed simply as a discontinuous cross-sectional expansion of a flow duct. In this case, a step extending substantially transversely to the main flow exists in the flow duct and this step acts as a separation edge for the flow. In the case of a sufficiently large velocity of the incident flow to this edge, periodic separation vortices form which extend parallel to this edge. The coherent vortex structures thus occurring can propagate substantially undamped in the flow direction. Should these periodic vortex structures reach the heat supply location--generally the flame--the periodic pressure fluctuations by which the vortices are manifested are amplified because of the resulting large increase in volume. As a result, thermo-acoustic vibrations of high amplitude occur and these concentrate a high level of vibration energy within a narrow frequency band and have potential for permanently damaging the structure of a heat generator.
It is precisely in modern gas turbine technology--where high flow velocities, high heat release rates and high pressures are present locally--that these thermo-acoustic vibrations play a decisive roll with respect to the reliable operation of the combustion chambers. Mastering them is therefore an essential precondition for the manufacture of gas turbine power stations and combined power stations.